Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (oleophilic areas) whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas.
Two basic types of lithographic printing plates are known. According to a first type, so called wet printing plates, both water or an aqueous dampening liquid and ink are applied to the plate surface that contains hydrophilic and hydrophobic areas. The hydrophilic areas will be soaked with water or the dampening liquid and are thereby rendered oleophobic while the hydrophobic areas will accept the ink. A second type of lithographic printing plates operates without the use of a dampening liquid and are called driographic printing plates. This type of printing plates comprise highly ink repellant areas and oleophilic areas. Generally the highly ink repellant areas are formed by a silicon layer.
Driographic printing plates can be prepared using a photographic material that is made image-wise receptive or repellant to ink upon photo-exposure of the photographic material. However heat mode recording materials, the surface of which can be made image-wise receptive or repellant to ink upon image-wise exposure to heat and/or subsequent development are also known for preparing driographic printing plates.
For example in DE-A-2512038 there is disclosed a heat mode recording material that comprises on a support carrying or having an oleophilic surface (i) a heat mode recording layer containing a self oxidizing binder e.g. nitrocellulose and a substance that is capable of converting radiation into heat e.g. carbon black and (ii) a non-hardened silicon layer as a surface layer. The disclosed heat mode recording material is image-wise exposed using a laser and is subsequently developed using a developing liquid that is capable of dissolving the silicon layer in the exposed areas. Subsequent to this development the silicon surface layer is cured. Due to the use of naphta as a developing liquid the process is ecologically disadvantageous. Further since the surface layer is not hardened the heat mode recording material may be easily damaged during handling.
FR-A-1.473.751 discloses a heat mode recording material comprising a substrate having an oleophilic surface, a layer containing nitrocellulose and carbon black and a silicon layer. After image-wise exposure using a laser the imaged areas are said to be rendered oleophilic. The decomposed silicon layer is not removed. Ink acceptance of the obtained plates is poor and the printing properties such as printing endurance and resolution of the copies is rather poor.
Research Disclosure 19201 of April 1980 discloses a heat mode recording material comprising a polyester film support provided with a bismuth layer as a heat mode recording layer and a silicon layer on top thereof. The disclosed heat mode recording material is imaged using an Argon laser and developed using hexane.
EP-A-573091 discloses a method for making a lithographic printing plate requiring a heat mode recording material comprising on a support having an oleophilic surface (i) a recording layer having a thickness of not more than 3 .mu.m and containing a substance capable of converting the laser beam radiation into heat and (ii) a cured oleophobic surface layer and wherein said recording layer and oleophobic surface layer may be the same layer.
From the above it can be seen that a number of proposals have been made for making a driographic printing plate using a heat mode recording material. All these plates have the disadvantage that they exhibit blocking. This results in difficulties in all steps wherein said materials are transported such as the winding-up during the fabrication, the format cutting and the packaging, the automatic loading on the press, the transport through the irradiation station, etc.